Analysis of corrosion pits in weld seams of internally threaded water-cooled wall tubes in subcritical boilers

For the problem of corrosion pits in the welds of internally threaded water-cooled wall tubes in a subcritical boiler, the failure mechanism under the synergistic effect of structural mutation and high-temperature hydrogen corrosion was revealed by means of laser confocal inspection, metallurgical examination, XRD analysis, and EDS analysis. The study shows that the fire-side weld of the front wall in the burnout wind region causes flow field distortion due to root protrusion and groove, which induces Cl ⁻ and Si ⁴⁺ /Al ³⁺ enrichment, forming pitting corrosion and subscale corrosion. at the same time, high-temperature hydrogen corrosion is triggered by the wall temperature of more than 400 ℃, microcracks at the near inner-wall grain boundaries are expanded and form mesh defects, and the scale layer exacerbates the local temperature rise to form the vicious circle of “high-temperature corrosion-crack expansion” and “high-temperature corrosion-crack expansion”. "Vicious cycle. A four-stage corrosion pit evolution model is proposed, and it is suggested to optimize the weld runner design and strictly control the water quality and heat load to block the failure chain.